This invention relates to a method and apparatus for testing optical components located in hazardous environments for the possibility of ignition and/or burning. The method and apparatus of the invention will allow testing laboratories to efficiently and accurately vary the irradiance of optical sources for certifying the safety of optical systems in hazardous (including classified) locations.
Fiber optic systems containing intense laser sources, such as laser diodes capable of producing several hundred milliwatts or more of power, are found in numerous industrial measurement, monitoring, and control applications. Besides the risk of human exposure, one safety concern with intense radiation sources is the potential for ignition of flammable gases, vapors, dusts, fibers, or flyings found in some industrial and hazardous locations.
Experiments have shown that optical devices, such as lasers, can provide sufficient energy to cause ignition of flammable gas/air or particulate/air mixtures, and, in some situations, burning of flammable materials present. One ignition process requires the conversion of optical energy to thermal energy by absorption in an appropriate target as shown in FIG. 15. Fiber optic systems placed within hazardous environments (i.e., areas in which there is a chance of explosion due to increased temperatures or flammable combinations of flammable gas/air or particulate/air mixtures) can be a potential ignition source. Faulty optical equipment, broken or stretched fiber optic cable, or improperly installed optical equipment can output optical beams above the critical or flash point in hazardous locations.
One particular application of optical technologies is the remote measurement of explosive methane gas in underground coal mines. Methane gas is often liberated during the mining process. In addition to methane-air mixtures, coal dust suspensions in air represent an explosion hazard and larger accumulations of coal dust on surfaces represent a smoldering fire hazard. Federal regulations require periodic methane measurements at the mining face, and abatement measures must be taken when methane concentrations exceed a threshold. Methane measurements often require elaborate safety precautions to prevent injury in the roof-fall prone face area. The difficulty in making remote methane measurements during extended cut operations has been cited as a safety concern by the United Mine Workers of America.
A remote measurement procedure has been proposed (see FIG. 13) in which an open laser beam passes through an area where both methane gas and coal dust are normally present. Federal regulations require that atmospheric monitoring systems used in gassy underground mines shall be intrinsically safe. However, intrinsic safety applies primarily to preventing electrical sparks or electrical heating ignition mechanisms. Little or no consideration has been given to optical ignition mechanisms because typical lasers used in mines have very low power. Also, the Mine Safety and Health Administration (MSHA) criteria for the evaluation and test of intrinsically safe apparatus and associated apparatus contain no specific guidance for optoelectronic components such as laser diodes.
One way to ensure the safety of a remote optical monitor is to limit the energy of the laser beam to below the critical duration and intensity which will result in ignition or burning in the proposed environment. Previous techniques to determine the duration and intensity needed for an optical component for ignition studies used external optical components to vary the irradiance (the optical power per area or optical power density) of the optical sources. Five cooperating laboratories in Europe investigated the conditions under which optical instruments using intense light sources (such as lasers) could operate safely in hazardous atmospheres containing vapors of various combustible products and/or combustible particulates. This study investigated the nature of light (i.e., coherence, intensity, wavelength, spectral width, and modulation), the characteristics of the illuminated particles (i.e., size, chemical and physical nature), and the nature of the gaseous environment. This study concluded that continuous wave devices radiating in the visible and near visible are not hazardous provided either the radiated power is less than 35 milliwatts, or the peak radiation flux is less than 5 milliwatts per square millimeter.
Details the these experimental techniques employed by the five cooperating laboratories is described in the report, xe2x80x9cOptical Techniques in Industrial Measurement: Safety in Hazardous Environments,xe2x80x9d European Commission, EUR 16011 EN, 1994. However, these techniques required very careful alignment and typically were designed for a single type optical source and were not easily adaptable to different optical sources. Accurately verifying the spot size of the output beam was also difficult, resulting in a time consuming test setup and the need for a variety of components (including different lenses and optical fibers) on hand to accommodate different optical sources. Also, the power values concluded to be safe may not be sufficient to provide remote monitoring in applications such as remote monitoring of methane in a mining operation.
There is a need for a method of testing optical systems, especially when installed in hazardous locations, to determine the risk of ignition. There is a need for a method of determining the maximum safe power output of an optical system to avoid the risk of ignition in a hazardous location. There is also a need for a method of determining the maximum safe power output of an optical system to avoid the risk of burning in hazardous locations where flammable materials are present. There is a need for an apparatus or system for testing optical systems for risk of ignition and burning which is easy to use with different optical sources. There is also a need for an optical test apparatus or system which provides for precise adjustment and verification of the output beam spot size. There is a need for an optical test apparatus or system which does not require use of additional components to accommodate different optical sources. There is also a need for a method to evaluate the failure mode when an optical fiber is stretched to the breaking point, with a concurrent reduction in fiber diameter, increasing the irradiance of escaping laser power. The present invention provides such methods and systems.
A method of determining the ignition characteristics of an optical source emitting optical power into a hazardous environment according to the invention includes providing a chamber and a tapered optical fiber having an input end and an output end, wherein the output end has a smaller diameter than the input end. The output end of the tapered fiber is disposed within the chamber and the input end of the tapered fiber is optically coupled to the optical source for receiving optical power therefrom. Power is first applied to the tapered fiber and the power output at the tapered fiber output end measured. Then a target is applied to the tapered fiber output end, and the chamber is filled with the desired gas/air mixture and the same power applied to the tapered fiber. After power is applied for a period of time, a determination is made whether or not the gas/air mixture ignited.
Apparatus for determining the ignition characteristics of an optical source emitting optical power into a hazardous environment according to the invention includes a chamber for receiving a known quantity of a hazardous material. A tapered optical fiber having an input end and an output end, wherein the output end has a smaller diameter than the input end, wherein the output end is disposed within the chamber. An optical coupler optically couples the tapered fiber input end to the optical source for receiving optical power therefrom. A target is attached to the output end of the tapered fiber and a video camera or pressure sensor is used to determine if ignition occurs when power is applied to the tapered fiber in the gas/air mixture filled chamber.
A method of determining the smoldering or burning characteristics of an optical source emitting optical power into a hazardous environment according to the invention includes measuring the temperature of the target at the tapered end of the fiber optic taper as power is applied to the taper.
The concept of conservation of brightness states that if light losses are negligible, the spatial and angular content of the light anywhere within or at either end of a fiber optic taper are described by:
Sini2 sin2(xcex8i)=Sono2 sin2(xcex8o)
where subscript i refers to input parameters, subscript o refers to output parameters, S is the cross-sectional area of the light distribution normal to the taper axis, xcex8 is the maximum angular extent of the light distribution, and n is the refractive index of the medium where xcex8 is measured. Since n sin(xcex8) is defined as the numerical aperture (NA) of the fiber and since Si/So=R2, where R2 is the taper diameter ratio, then NAo/NAi=R.
These expressions show that fiber optic tapers are useful for transforming spatially structured input beams (such as those produced by different optical sources) into a spatially uniform output spot. Therefore, a single taper can be used to more efficiently guide optical energy from a larger variety of input sources while maintaining uniform output characteristics than possible from a non-tapered fiber. The uniform output characteristics and well-defined taper dimensions allows more accurate measurement of irradiance than practical with the external optical components used by the prior art. Accurate measurements are critical in the safety certification process. Also, by transforming the beam within the taper, external components are not necessary to adjust the power or focus the output spot, simplifying the test setup, reducing component inventory, and reducing the risk of human exposure to the optical beam.
These expressions also show (and as supported by experimental evidence obtained by the inventors) that a fiber optic taper increases the irradiance of an optical source. Thus, a method and apparatus employing a taper allows a controlled way of simulating enhanced irradiance that may occur in a broken optical fiber.
Beam transformation within the fiber eliminates the need for external optical components. Fiber tapers require less careful alignment than external components, simplifying the test setup and reducing setup time. Fiber tapers can accept a wide range of optical sources while maintaining uniform output characteristics, reducing component inventory required if using external components. The well-defined output characteristics of the method of the invention will allow accurate measurement of optical beam properties more easily and reliably than practical with external components.
The method and apparatus of the invention will allow testing laboratories to efficiently and accurately vary the irradiance of optical sources for certifying the safety of optical systems in hazardous (including classified) locations. The irradiance enhancement demonstrated by the method of the invention will allow a controlled method for simulating potential irradiance enhancement of a broken optical fiber. The smaller cross-sectional area of the taper reduces thermal conductivity effects, representing more severe testing conditions of fiber-optical systems than using untapered fibers.